Adaptive Wordline Programming Bias of a Phase Change Memory

ABSTRACT

The leakage current and power consumption of phase change memories may be reduced using adaptive word line biasing. Depending on the particular voltage applied to the bitline of a programmed cell, the word lines of unselected cells may vary correspondingly. In some embodiments, the word line voltage may be caused to match the bitline voltage of the programmed cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/963,717, filed on Dec. 9, 2010, which is a divisional of U.S. patentapplication Ser. No. 11/901,493, filed on Sep. 18, 2007, which issued asU.S. Pat. No. 7,885,099.

BACKGROUND

This relates generally to phase change memories that use chalcogenidematerials.

Phase change memory devices use phase change materials, i.e., materialsthat may be electrically switched between a generally amorphous and agenerally crystalline state, for electronic memory application. One typeof memory element utilizes a phase change material that may be, in oneapplication, electrically switched between a structural state ofgenerally amorphous and generally crystalline local order or betweendifferent detectable states of local order across the entire spectrumbetween completely amorphous and completely crystalline states. Thestate of the phase change materials is also non-volatile in that, whenset in either a crystalline, semi-crystalline, amorphous, orsemi-amorphous state representing a resistance value, that value isretained until changed by another programming event, as that valuerepresents a phase or physical state of the material (e.g., crystallineor amorphous). The state is unaffected by removing electrical power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit depiction of a phase change memory in accordancewith one embodiment of the present invention;

FIG. 2 is a depiction of a digital-to-analog converter for use with aphase change memory cell in accordance with one embodiment of thepresent invention;

FIG. 3 is a flow diagram in accordance with one embodiment of thepresent invention; and

FIG. 4 is a system depiction in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION

In connection with the programming of phase change memories, it isdesirable to limit the conduction of unselected cells on unselected rowsand columns. Conduction by these unselected cells is unnecessary andcreates adverse consequences. For example, this conduction increases thepower consumption of the overall memory. Given the fact that phasechange memories can have a large number of cells, conduction byunselected cells may be a significant source of power consumption.

Generally, the amount of bias that needs to be applied to unselectedcells to turn them off is a function of the bias supplied to theselected cells. In some cases, the bias applied to the selected cellsmay vary, for example, due to process variations. When this variationhappens, it may be advantageous to vary the bias on the unselectedcells.

In accordance with some embodiments, the bias on the unselected cellsduring programming may be varied to match the bias on the selectedcells.

In the embodiment shown in FIG. 1, each cell A, B, C, or D may include aphase change memory element 14 and a bipolar junction transistor 16.However, other embodiments are contemplated and the present invention isnot limited to the specific cell design depicted.

Zero volts applied to the base of the bipolar junction transistors 16for the selected word line allows the emitter base junction to forwardbias, allowing programming current to pass through the memory element14. The bitline 12 uses a relatively high voltage, in this case from 3.5to 4.5 volts. This voltage may vary depending on the cell's behaviorwith respect to process induced variations. One cell may require, inthis example, 3.5 volts, while another cell may require a bitlineprogramming voltage of 4.5 volts and others may require something inbetween.

A portion of a phase change memory array 10 is depicted. The array 10includes columns 12 and rows 14. In one embodiment, a word line or rowbias of 3.5 to 4.5 volts is applied by a decoder 25 to the unselectedcells A and B on an unselected row 14, while a bias of zero volts isapplied to the row 14 including the selected cell D. At the same time,an unselected bitline 12 may have 0.3 volts applied by the bitlinedecoder 55, while the selected bitline or column line 12 may have 3.5 to4.5 volts in this example. Of course, the voltages applied arecompletely hypothetical and the present invention is in no way limitedto specific applied voltages.

However, it may be appreciated that if the selected cell D is exposed toa bitline bias varying from 3.5 to 4.5 volts, this may complicate thebias that needs to be applied to the unselected cells, such as the cellsA and B in unselected rows. They too must received a word line bias thatvaries according to the bias applied to the bitline of the selected cellin order to turn them off. Thus, the unselected word line voltage isrelated to the selected bitline voltage and, therefore, may be adjustedaccording to the changes, from cell to cell, in the bitline programmingvoltage.

Referring to FIG. 2, in accordance with one embodiment, the word linedecoder 25 includes a digital-to-analog converter 18 coupled to the wordlines 14. The digital-to-analog converter 18 may include a series ofresistors 60 of different resistance values so that at nodes between theresistors 60, different output voltages, indicated as VPX, can beprovided selectably. In other words, a digital voltage selectiondetermines the output node. The voltage VPX from that output node issupplied to an amplifier 20 and then to the selected word line 14, and,finally, to the base of each bipolar junction transistor 16 in each cellA-D.

It may be desirable to have as low a leakage on the unselected cells aspossible so that the unselected word lines have a bias equal to the biason the selected bitlines during programming. In some embodiments, it mayalso be desirable to avoid over-programming the cells. In general, it isdesirable to step the program voltage up in increments so that the cellswhich require lower voltages do not see the higher voltages.

An adaptive programming approach, shown in FIG. 3, causes the unselectedword line bias to adapt the selected bitline bias. A voltage may “adaptto” another voltage without precisely matching or following the othervoltage precisely by moving in the same direction as the other voltage.In some embodiments, the adaptive programming approach may beimplemented by software stored in the control 75. The control 75 may bea controller with an internal storage in one embodiment. On a firstpulse of N (e.g., where N=3), a lowest program voltage is applied, inthis example 3.5 volts (block 26). The debias voltage follows with 3.5volts (block 24). A verify is performed (block 28), and those cellswhich pass (diamond 38) do not see a further programming pulse (block34). Cells which do not pass continue with the algorithm and see anelevated voltage (e.g., 4.0 volts) with the debias voltage following(block 32). Those that still do not pass see the highest voltage (e.g.,4.5) volts as the debias voltage on the word lines (block 40) and as thebitline programming voltage (block 42). In this example, where N is 3,three pulses are available, 3.5, 4.0, and 4.5 volts, but these numberswould be different for different designs, technologies, andarchitectures. If the cell still does not pass, it is failed (block 48).

It may be advantageous to avoid leaving the unselected or debiased wordline voltage always at the highest level, shutting off the B cells. Thedisadvantage of doing this is that the cells A see a higher reverse biasvoltage and, thereby, would leak more, impacting the background currentconsumption during programming. The A cells are important since the vastmajority of the cells in the array are biased in this way. A smallfraction of cells may require the highest voltage, so statistically itis not often that the third pulse will be applied at all. Since on eachiteration the higher voltages are applied only for a fraction of theoverall run time, the elevated voltage and, thus, the background leakageto the A cells contributes little to overall energy consumption.

Programming of a chalcogenide material within a cell to alter the stateor phase of the material may be accomplished by generating a voltagepotential across the memory element. When the voltage potential isgreater than the threshold voltages of memory element, then anelectrical current may flow through the chalcogenide material inresponse to the applied voltage potentials, and may result in heating ofthe chalcogenide material.

This heating may alter the memory state or phase of the chalcogenidematerial. Altering the phase or state of the chalcogenide material mayalter the electrical characteristic of memory material, e.g., theresistance of the material may be altered by altering the phase of thememory material. Memory material may also be referred to as aprogrammable resistive material.

In the “reset” state, memory material may be in an amorphous orsemi-amorphous state and in the “set” state, memory material may be in acrystalline or semi-crystalline state. Both “reset” and “set” states canexist without any energy (electrical, optical, mechanical) applied tobistable chalcogenide. The resistance of memory material in theamorphous or semi-amorphous state may be greater than the resistance ofmemory material in the crystalline or semi-crystalline state. It is tobe appreciated that the association of reset and set with amorphous andcrystalline states, respectively, is a convention and that at least anopposite convention may be adopted.

Using electrical current, memory material may be heated to a relativelyhigher temperature to amorphosize memory material and “reset” memorymaterial (e.g., program memory material to a logic “0” value). Heatingthe volume of memory material to a relatively lower crystallizationtemperature may crystallize memory material and “set” memory material(e.g., program memory material to a logic “1” value). Variousresistances of memory material may be achieved to store information byvarying the amount of current flow and duration through the volume ofmemory material.

Turning to FIG. 4, a portion of a system 500 in accordance with anembodiment of the present invention is described. System 500 may be usedin wireless or mobile devices such as, for example, a personal digitalassistant (PDA), a laptop or portable computer with wireless capability,a web tablet, a wireless telephone, a pager, an instant messagingdevice, a digital music player, a digital camera, or other devices thatmay be adapted to transmit and/or receive information wirelessly. System500 may be used in any of the following systems: a wireless local areanetwork (WLAN) system, a wireless personal area network (WPAN) system, acellular network, although the scope of the present invention is notlimited in this respect.

System 500 may include a controller 510, an input/output (I/O) device520 (e.g. a keypad, display), static random access memory (SRAM) 560, amemory 530, and a wireless interface 540 coupled to each other via a bus550. A battery 580 may be used in some embodiments. It should be notedthat the scope of the present invention is not limited to embodimentshaving any or all of these components.

Controller 510 may comprise, for example, one or more microprocessors,digital signal processors, microcontrollers, or the like. Memory 530 maybe used to store messages transmitted to or by system 500. Memory 530may also optionally be used to store instructions that are executed bycontroller 510 during the operation of system 500, and may be used tostore user data. Memory 530 may be provided by one or more differenttypes of memory. For example, memory 530 may comprise any type of randomaccess memory, a volatile memory, a non-volatile memory such as a flashmemory and/or a memory such as memory discussed herein.

I/O device 520 may be used by a user to generate a message. System 500may use wireless interface 540 to transmit and receive messages to andfrom a wireless communication network with a radio frequency (RF)signal. Examples of wireless interface 540 may include an antenna or awireless transceiver, although the scope of the present invention is notlimited in this respect.

References throughout this specification to “one embodiment” or “anembodiment” mean that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneimplementation encompassed within the present invention. Thus,appearances of the phrase “one embodiment” or “in an embodiment” are notnecessarily referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be instituted inother suitable forms other than the particular embodiment illustratedand all such forms may be encompassed within the claims of the presentapplication.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: causing a cell bias during programming of anunselected phase change memory cell to adapt to a cell bias applied to aselected phase change memory cell during programming.
 2. The method ofclaim 1 wherein causing a cell bias includes causing a word lineprogramming voltage of an unselected phase change memory cell to adaptto a programming voltage applied to the bitline of the selected cellduring programming.
 3. The method of claim 2 including programming witha first lower voltage and debiasing an unselected word line with saidfirst lower voltage.
 4. The method of claim 3 including programming witha first higher voltage, higher than said first lower voltage, if thecell is not programmed by said first lower voltage.
 5. The method ofclaim 4 including using a second higher voltage, higher than said firsthigher voltage, to program a cell not programmed by said first highervoltage.
 6. The method of claim 2 including using a digital-to-analogconverter and a resistor array to apply different word line debiasingvoltages.